1. Field of the Invention
The present invention relates generally to methods of fracturing formations in wells and, more particularly, to methods of hydraulically fracturing and propping a formation after the formation is initially fractured.
2. Setting of the Invention
Oil and gas accumulations usually occur in porous and permeable underground rock formations. In order to produce the oil and gas contained in a formation, a well is drilled into the formation. The oil and gas may be contained in the porosity or pore spaces of the formation, the pore spaces being hydraulically connected by means of permeability or interconnecting channels between the pore spaces. After the well is drilled into the formation, oil and gas are displaced to the wellbore by means of fluid expansion, natural or artificial fluid displacement, gravity drainage, capillary expulsion, etc. These various processes may work together or independently to draw out the hydrocarbons into the wellbore through existing flow channels. In many instances, however, production of the well may be impaired by drilling fluids that enter into and plug the flow channels, or by insufficient natural channels leading into the particular borehole. Either case may result in a noncommercial well caused by the low permeability. The problem then becomes one of treating the formation in a manner which will increase the ability of a formation rock to conduct fluid into the well.
Hydraulic fracturing is a widely used well stimulation technique designed to increase the productivity of a well by creating fractures in the formation surrounding the well. The technique normally involves two basic steps: (1) injecting a fluid into the formation at a rate and pressure sufficient to propagate a fracture adjacent the well and (2) placing propping agent particles in the fracture to maintain it in a propped condition when the applied pressure is relieved. During the propagation step of the treatment, the fracturing fluid must have properties which provide the proper wedging effect for creating a fracture of the desired geometry. These properties relate to the fluid loss characteristic and the viscosity of the fluid employed as the fracturing medium.
It has long been known that the fluid efficiency of a fracturing fluid must be high if fractures of reasonable lengths and widths are to be obtained. Fluid efficiency, as used in fracturing operations, is defined as the percent of injected fluid which remains within the fracture and is a measure of the fluid loss characteristic of the fluid. Many fluids tend to leak off rapidly into the formation matrix and therefore provide low fluid efficiencies. The fluid efficiency can be improved by the addition of fluid loss control additives, which are generally finely divided inert materials such as silica flour, talc, calcium carbonate, and the like.
In the design of a fracturing fluid, the effect of the fluid on pumping requirements must be considered. The wedging effect described above must be achieved without excessive friction loss in the well conduit leading to the formation. High friction losses limit the available power deliverable to the formation, increase the power requirements of injection facilities, and can generate pressures which approach the pressure limits of the wellbore tubing.
During the proppant placement phase of the treatment, the fracturing fluid acts as a carrier for the propping agent particles. The fluid must thus be capable of suspending large concentrations of particles for long transport distances.
Finally, the fracturing fluid should not damage the formation. This generally means that the fluid at formation conditions must possess sufficient mobility in the formation matrix and the propped fracture to be producible into the well along with formation fluids. This operation is referred to as well clean-up. Such fluid loss additives as silica flour, talc, calcium carbonate causes clogging of the fracture.
The characteristics of an ideal fracturing fluid, then can be summarized as follows: low friction loss in the well conduit; low fluid loss in the fracture during fracturing; controllable friction loss in the fracture; good proppant suspension capabilities; and high fluid loss in the fracture upon shut-in.
3. Publications
U.S. Pat. No. 4,109,721, Marion L. Slusser, issued on Aug. 29, 1978, and entitled "Method of Proppant Placement in Hydraulic Fracturing Treatment" is a relevant publication. In this patent, a method is disclosed for refracturing wells which have previously been fractured and a proppant pack deposited therein, a slug of liquid containing a sealant, normally fluid loss additives and desirably also a viscosifier, is injected via the well and into the formation to deposit the sealant or fluid loss additives along the upper surface of the previously deposited proppant pack and form a seal overlying this proppant pack. After sealing the upper surface of the previously deposited proppant pack, the well is refractured by injecting a high fluid loss fracturing fluid containing proppants suspended therein via the well into the formation to extend the fracture further in the formation to extend the fracture further in the formation and prop the extended portion thereof.
U.S. Pat. No. 3,710,865, Othar M. Kiel, issued on Jan. 16, 1973 and entitled "Method of Fracturing Subterranean Formations Using Oil in Water Emulsions" is also a relevant publication. In the Kiel patent, a fracturing method using an emulsion fracturing fluid is made up of a hydrocarbon phase and a gelled water phase. The viscosity of the emulsion at 70.degree. F. was preferred to be 100 centipoises.
The present invention provides a method for fracturing a formation during which there is leakoff control of the fracturing fluid, i.e., a very low fluid loss coefficient and efficient proppant transportability. Overall, this method results in an improved efficiency of fracture treatment by creating more propped fracture area for less injected fluid volume.